(1) Field of the Invention
This invention relates to a heat welding apparatus, and more specifically, to an apparatus for bonding a heat-meltable sheet rapidly to a substrate by utilizing near infrared radiation.
(2) Description of the Prior Art
In the prior art, a dash panel for separating the passenger compartment of an automobile from an engine compartment therein is made by bonding an asphalt sheet having a thickness of several millimeters as a soundproof antivibratory material to a steel sheet. Methods for bonding which have previously gained practical acceptance include bonding by an adhesive, heat welding by the Joule's heat generated by a high-frequency induction current, or heat welding using far infrared rays.
The method of bonding with an adhesive, however, has the defect that the adhesive is expensive, expert skills are required for coating the adhesive and bonding a fusible sheet, the number of steps is large and the method is time-consuming. The high frequency welding method has a reduced cost, but has the defect that non-uniformity in welding tends to occur, and by re-heating in the subsequent coating and drying steps, the asphalt sheet is sometimes peeled or melted. The heat welding method using far infrared rays also has defects. The temperature elevating time at the time of starting the heating of a far infrared ray source is relatively long. Hence, when a sheet welding apparatus is directly connected to a production line, the far infrared ray source must be started well before the start of operation of the production line. When for some reason, the entire production line or that part of the line to which the sheet welding apparatus is connected stops, the far infrared ray source is taken out of operation. When, however, the operation of the line is resumed the workpieces cannot be immediately heated, and the operational efficiency of the entire production line is reduced. Hence, the sheet welding apparatus cannot be conveniently incorporated in the production line.
The present inventors have found that by utilizing near infrared radiation for the heat bonding of asphalt sheet to a dash panel as described above, the heat welding operation is completed within a short period of time, and consequently, the operation of heat welding the asphalt sheet to the dash panel can be directly connected to a production line such as an automobile assembly line.